1. Field of the Invention
The present invention relates to a semiconductor laser control device that controls the optical output of a semiconductor laser device used as a light source of a photoelectronic system such as a laser printer, an optical disk device or an optical communication system.
2. Description of the Related Art
The semiconductor laser device has been widely used lately in various technical fields as a light source of an optical disk device and a laser printer etc. However, the optical output characteristic of the semiconductor laser device is strongly influenced by the ambient temperature. To cope with this, various types of APC (Automatic Power Control) circuits have been proposed in order to obtain a desirable optical output from the laser device. There are three essential types of the APC circuit.
The first type of the APC circuit is arranged in such a way that the optical output from the semiconductor laser device is monitored by a light receiving element and that a photoelectric negative feed back loop is formed to control the forward current of the semiconductor laser to equalize the detection output of the light receiving element to the light emitting level of the command signal so that the optical output from the laser device is controlled to be a desirable value.
The second type of the APC circuit is arranged in such a way that, in a power setting period, the optical output from the semiconductor laser device is monitored by a light receiving element so as to control the forward current of the laser so that the detection output of the light receiving element becomes equal to the level of the light emitting command signal and that, in a period other than the power setting period, the laser device is controlled so that the forward current of the laser is modulated by information to maintain a set value.
The third type of the APC circuit is arranged in such a way that the temperature of the laser device is measured so as to control the forward current of the laser in response to the measured temperature or to maintain the temperature constant so that a desired optical output can be obtained from the semiconductor laser device.
Further, there is a fourth type of the APC circuit which is superior to the three types of the control circuits mentioned above. The fourth type control circuit is arranged in such a way that the optical output from the semiconductor laser device is monitored by a light receiving element, that a photoelectric negative feed back loop is formed to control the forward current of the laser so that the detection output of the light receiving element becomes equal to the level of the light emitting command signal and that a current converter is provided for converting the light emitting command signal to the forward current of the semiconductor laser so that the laser device is controlled in response to the sum of or difference between the current generated from the converter and the control current for the negative feed back loop.
However, in accordance with the fourth type of control circuit, when the differential quantum efficiency is changed, the cross frequency of the photoelectric negative feed back loop is also changed in the open loop state thereof. As a result, the control speed of the feed back loop becomes different from the preset value, which lowers the control speed and accuracy of the semiconductor laser output and impairs the resolution of the laser and which makes the laser device easy to be influenced by disturbance.